1. Field of the Invention
The present invention relates generally to an automatic automotive transmission and more specifically to a line pressure control arrangement for such a transmission.
2. Description of the Prior Art
An automatic transmission Service Manual (publication A261C07) issued in March 1987 relating to the so called RE4R01A type transmission describes an arrangement which utilizes tabled data for controlling the line pressure during shifting and non-shifting modes of transmission operation. In this arrangement the engine throttle valve opening degree is used to determine the duty cycle of a signal used to control a line pressure control solenoid valve.
However, the solenoids used for the purposes of line pressure control and the friction elements used in a given transmission, inevitably exhibit a unit to unit variation and/or exhibit a change in characteristics with the passing of time due to wear and the like deterioration. These changes and/or unit to unit variations cannot be compensated as it not possible to change the line pressure control valve duty cycle.
Accordingly, the control characteristics tend to deteriorate to the point of permitting strong shift shocks and/or excessive friction element slippage which shortens the working life of the friction elements per se.
As shown in the timing chart of FIG. 9, when the engine is operating at very low throttle settings and the basic level of the line pressure is reduced, and the transmission and a shift control solenoid is switched from an energized condition to a de-energized (at time t1) in a manner to produce a 1-2 upshift; the level of the pressure which is supplied to a given friction element to induce the engagement of the same, varies as indicated by the solid line trace. In response to this the rotational speed ratio (viz., Nt/No) which occurs between the transmission input and output shafts varies, as shown by the solid line trace, from the value produced by first speed operation (first gear) to that which is produced by second speed (second gear) operation. On the other hand, if the line pressure is higher, the pressure development and the change in the rotation speed ratio Nt/No change in the manner indicated by the broken line.
In accordance with the above, the torque which appears on the output shaft of the transmission varies in the case of the low line pressure level in the manner indicated by the solid line and in the manner indicated by the broken line in the case of the higher level.
Accordingly, it is possible to determine from the inertia phase period (T2) if the above mentioned wear and unit to unit variation factors have exerted an influence on the operational characteristics of the transmission for the instant level of line pressure.